Which composer is associated with the first known mass?

The first known mass is attributed to the composer Guillaume de Machaut, who was active during the 14th century. His work, the "Messe de Nostre Dame," is significant not only for its historical importance as one of the earliest complete settings of the Ordinary of the Mass by a single composer but also for its intricate use of polyphony, which was a hallmark of the late medieval music period. Machaut's compositions represent a transition from the medieval to the Renaissance style, showcasing early developments in musical notation and choral arrangement that would influence generations of composers after him. This unique context and his pioneering role in sacred music history make him the correct choice associated with the first known mass.